Continuous blood gas and pH monitoring devices developed over the past 20 years have failed primarily due to lack of stability and the associated dependency on the clinical laboratory for calibration. Continuous electrolyte monitor developments, of particular importance for neonatal and infant monitoring applications, have not been attempted. The goal of this project is to develop a commercially viable continuous blood gas, pH, and electrolyte monitor which utilizes a self-calibration mechanism and existing sensor technology and can maintain levels of accuracy and reliability equal to and independent of the clinical laboratory. Concept feasibility for monitoring by this principle as well as the supporting sensor and membrane technology for gases and ions has been established in vitro experiments using human blood with anticoagulants. What remains to be demonstrated is that the membranes developed in the Phase I can perform satisfactorily in non heparinized patients. Therefore, this proposal is directed toward developing a prototype which can safely be used clinically to establish the reliability of the method, including the resistance of the membrane to protein fouling and clotting and demonstrate that it meets the specifications for a practical blood gas, pH and electrolyte monitor.